issue-1350: multitablet filesystems api, follower configuration code, automatic session creation in followers, node creation in followers via requests to leader, service uts

[qkrorlqr]

Implementing only happy path in the first version. Leaving TODOs in the places where I took shortcuts.

Private API:

• ConfigureFollowers - tells the tablet that it has "follower" tablets - effectively, followers are file node shards
• ConfigureAsFollower - tells the follower tablet that it is the shard with ShardNo=X

ShardNo is needed to split the NodeId and HandleId spaces between the leader and the followers so that they do not intersect. Follower tablets store regular files (attributes and data) directly under RootNode, names in NodeRefs are guids. Leader tablet stores all the other node types (the most important thing which it stores is the directory structure).

The current PR describes the intended purpose of all these changes via the UTs in service_ut.cpp. My next PR will contain the usage of the things implemented in this PR in TStorageServiceActor.

I left several TODOs regarding the "unhappy" path. Will implement them after successful performance tests of the happy path. Will implement follower metrics aggregation in the leader after that as well.

1350

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:red_circle: linux-x86_64-relwithdebinfo: some tests FAILED for commit ff2c0609e1c169b5f5c8bcaee734f0c4851b04ef.	TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
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:green_circle: linux-x86_64-relwithdebinfo: all tests PASSED for commit 0a3bd4e7de1b9afecbecb6b9a98beeed34d82e8b.	TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
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:green_circle: linux-x86_64-relwithdebinfo: all tests PASSED for commit 40ea3a3c62940f0a84965274c216c9ec7273e588.	TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
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:green_circle: linux-x86_64-relwithdebinfo: all tests PASSED for commit 30ef792c75bc42072a0583ea9594a6a93c548464.	TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
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:green_circle: linux-x86_64-relwithdebinfo: all tests PASSED for commit 2ec9ab040933e7f7993f4cb0f38fc4c7d04be4b6.	TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
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:green_circle: linux-x86_64-relwithdebinfo: all tests PASSED for commit 9c9b5965d390ce7da0261f5fea1124655c2b7ead.	TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
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:green_circle: linux-x86_64-relwithdebinfo: all tests PASSED for commit bb92a37132e65270d57ee6f90b37fbd3a1098178.	TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
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